Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/book/build.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -e
+
+cd "$(dirname "$0")"
+
+if [ -f ./mdbook ]; then
+    ./mdbook build
+else
+    mdbook build
+fi
+
+if [ ! -d node_modules ]; then
+    npm install
+fi
+
+./build.js